With increases in technology developments and demands for mobile devices, demands for secondary batteries as an energy source have rapidly increased, and among such secondary batteries, lithium secondary batteries having high energy density and voltage have been commercialized and widely used.
In a lithium secondary battery, lithium metal oxides are used as a positive electrode active material, and lithium metal, lithium alloys, crystalline and amorphous carbon or carbon complexes are used as a negative electrode active material. A secondary battery is manufactured by coating the active material on a collector to suitable thickness and length or coating the active material itself in a film form, and winding or laminating the active material with a separator that is an insulator to form an electrode group, placing the result in a can or a container similar thereto, and then injecting a liquid electrolyte.
Such a lithium secondary battery experiences charge and discharge while repeating intercalation and deintercalation of lithium ions from a lithium metal oxide of a positive electrode to a graphite electrode of a negative electrode. Herein, the lithium reacts with a carbon electrode due to high reactivity, and forms a film on the negative electrode surface by producing Li2CO3, LiO, LiOH and the like. Such a film is referred to as a solid electrolyte interface (SEI) film, and the SEI film formed at the beginning of charge prevents a reaction of lithium ions with a carbon negative electrode or other materials while charging and discharging. In addition, the SEI film performs a role of an ion tunnel and passes only lithium ions. This ion tunnel solvates lithium ions and performs a role of preventing the collapse of the carbon negative electrode structure by a high molecular weight organic solvent of a liquid electrolyte moving together being co-intercalated to the carbon negative electrode.
Accordingly, a solid SEI film needs to be formed on a negative electrode of a lithium secondary battery in order to enhance a high temperature cycle and a low temperature output of the lithium secondary battery. Once the SEI film is formed at the initial charge, the film prevents reaction of lithium ions with a negative electrode or other materials when repeating charge and discharge by battery use thereafter, and performs a role of an ion tunnel between a liquid electrolyte and a negative electrode passing only lithium ions.
In existing technologies, a low temperature output property enhancement has been difficult to be expected in a liquid electrolyte that does not comprise liquid electrolyte additives or comprises electrolyte additives having poor properties due to the formation of a non-uniform SEI film. Moreover, even when liquid electrode additives are included but the amounts of the additives to be added are not adjusted to required amounts, there is a problem in that a positive electrode surface is decomposed in a high temperature reaction due to the liquid electrolyte additives, or the liquid electrolyte causes an oxidation reaction and ultimately, irreversible capacity of a secondary battery increases and an output property declines.